1. Field of the Invention
The present invention relates to a method and apparatus of concentrating chemicals during semiconductor device manufacturing processes and, more particularly, to a method and apparatus of concentrating chemicals which are used during a pretreatment of a sample prior to analyzing the chemicals for a semiconductor device in a chemical analytical instrument.
2. Description of the Related Art
A micro contaminant existing in constituent layers of a semiconductor device has a greater effect on the device characteristics as the size of the device is reduced due to the higher integration of a semiconductor device. Accordingly, the goal is to remove as many of the micro contaminants as possible throughout the fabrication process of a semiconductor device. A wet cleaning process is the most typical measure taken to reduce or eliminate contamination of a wafer.
Chemicals such as fluoric acid, nitric acid, sulfuric acid and hydrogen peroxide solution, etc. are chiefly used in a wet cleaning process, and those chemicals are required to have a high degree of purity so as to prevent a wafer from being recontaminated. Therefore, it is necessary to perform a quantitative and qualitative analysis of the micro contaminants in the chemicals. Contaminants may include such heavy metals as iron(Fe), aluminum(Al) and copper(Cu), etc. and ions of sodium(Na+), ammonium(NH.sub.4.sup.+), nitrate(NO.sub.3.sup.-) and chloride(Cl.sup.-), etc. To analyze these contaminants, analytical instruments such as a graphite reactor atomic absorption spectrometer (GFAAS), an induced coupled plasma mass spectroscope (ICP-MS) and ion chromatography (IC) are mainly used.
However, since the chemicals used in a wet cleaning process are highly pure, it is difficult to make a quantitative analysis with the usual analytical instruments due to the minimum limitations of detection.
Accordingly, to ensure an effective quantitative analysis of chemicals used for semiconductor device manufacturing, a chemical concentration method is applied to increase the relative concentrations of the micro contaminants in a matrix by vaporizing the chemicals with the contaminants and reducing the amounts of the matrix.
Well-known chemical concentration methods include isopiestic distillation, below-boiling-point distillation and vacuum distillation for attaining a highly pure acid.
Isopiestic distillation is a method of concentrating the chemicals of a sample on the basis of the difference in vapor pressures. Isopiestic distillation is applicable to the concentration of acids having a high vapor pressure, for example, hydrochloric acid, rather than acids having a low vapor pressure, for example, nitric acid and sulfuric acid. Accordingly, it is not suitable for the pretreatment of samples.
Below-boiling-point distillation is performed by a closed system comprising two heated and cooled bottles perpendicularly installed for sample-concentration. This distillation method is applicable to the concentration of fluoric acid having a low boiling point, but it is not suitable for sulfuric acid having a high boiling point.
Another below-boiling-point distillation method is disclosed in Edwin C. Kuehner et al., "Production and Analysis of Special High-Purity Acids Purified by Sub-Boiling Distillation", Journal of Analytical Chemistry, vol. 44, No. 12, p. 2050 (1972). While the method disclosed by Kuehner is suitable for a concentration of acids having a high boiling point such as sulfuric acid, it is not suitable for micro analysis because of the constructive problems of the instrument. In particular, it takes a long time to concentrate a sample and it is easy to contaminate a sample when provided to the analytical instrument.
Vacuum distillation can be used for acids having a high vapor pressure but is not applicable to micro analysis because the sample can be contaminated through the wall of a container when the container is heated.
In another method of concentrating chemicals, a quartz beaker containing a sample is placed in a Pyrex box through which nitrogen gas filtered by a HEPA filter flows. The box is heated by radiating infrared rays from above the box and setting a high-temperature plate below the box so as to concentrate the sample.
The above-described method reduces the contamination of a sample from the surrounding environment and it also reduces the danger of damaging an air cleaner by catching chemicals in a workroom. However, the method is problematic in that the sample may be lost through the wall of the beaker and the sample may be contaminated because the Pyrex or quartz beaker is directly heated.